RE: [PATCH 0/4] Make xfrm usable by 32-bit programs

From: Kevin Cernekee

Sent: 21 January 2017 00:05
Several of the xfrm netlink and setsockopt() interfaces are not usable
from a 32-bit binary running on a 64-bit kernel due to struct padding
differences.  This has been the case for many, many years[0].  This
patch series deprecates the broken netlink messages and replaces them
with packed structs that are compatible between 64-bit and 32-bit
programs.

Do you mean '__packed' or just structures with no holes?
The latter is really the best.
You can add a compile time assert on the structure sizes to ensure
that the user API is never accidentally broken.

I can't imagine any reason to change the 64bit structures except
(maybe) to make any padding explicit.

It retains support for legacy user programs (i.e. anything
that is currently working today), and allows legacy support to be
compiled out via CONFIG_XFRM_USER_LEGACY if it becomes unnecessary in
the future.

...

Isn't that effectively the same as doing a compat layer?
Otherwise you can't build a 32bit app that will work with an 'old'
32bit kernel and a new 64bit one.

Provided you've got the length of the user's buffer the compat code
ought to be trivial (if tedious).

	David